Reference solution dynamo. Top: normalised volume averages of the squared horizontal magnetic field components, (blue) and (red), as functions of time; the black dotted line shows . Middle: horizontally averaged horizontal magnetic fields (normalised by the equipartition field strength) as functions of z and time. Bottom: mid-layer kinetic (black) and magnetic (red) energy spectra, normalised by the average kinetic energy at z_m at the end of the dynamo run. The inset highlights the low wavenumber behaviour of the spectra, using a non-logarithmic abscissa to include k∕k₁ = 0.